In an increasing number of cases, distributed power sources such as solar power generation (or sometimes is called photovoltaic generation), wind power generation (or sometimes is called wind force power generation) or fuel cells are being linked with power systems: this gives rise to concern regarding possible fluctuation of the system voltage. In particular, further increase in domestic solar power generation is anticipated. When a large amount of power is generated and the load is small, large reverse power flows may occur, causing elevation or rising of the voltage of the distribution network, which may therefore depart from the rated value of the system voltage.
Conventionally, such voltage elevation is suppressed by throttling the generated power. However, with this method, the power that could be generated is throttled, so this is wasteful in energetic terms. If a reactive power compensator is employed to inject reactive power into the system, the system voltage can be lowered without giving rise to wastage of energy, so distributed power sources such as solar power can deliver the full power of which they are capable. However, conventional reactive power compensators were of large weight and volume and were therefore mostly installed at the delivery terminal side, such as an electrical substation. In order to cope with elevation of the system voltage of the distribution network resulting from the use of distributed power sources such as domestic solar power generation, there is therefore a demand for a reactive power compensator of small size that can be installed even in narrow residential streets.
One means of reducing the size of a reactive power compensator is the method of reducing the size of the AC filter reactor (Reactor means here inductor), by converting the voltage waveform that is output by the semiconductor power inverter section to a voltage waveform with fewer higher harmonics, which is closer to a sine wave. Conventionally, in order to output a voltage waveform with few higher harmonics, the method was adopted of creating an interconnection voltage waveform by connecting in series multiple PWM (Pulse Width Modulation) inverters that output pulse width-modulated voltage waveforms of small voltage width. The size of the AC filter reactor can be reduced by reducing the harmonics of the output voltage by superimposing pulse outputs of small voltage width. Also, ample withstand-voltage can be achieved by increasing the number of multiple PWM inverter stages that are connected in series: in this way, reduction in size can be obtained without employing a system-interconnection transformer.
Such an arrangement is described in the Japanese technical reference: “Operation Verification using 6.6 kV Transformerless Cascade PWM STATCOM-Three-Phase 200V 10 kVA Mini Model” (Journal of the Institute of Electrical Engineers of Japan Industrial Applications Section, 2007, Vol. 127, No. 8 pp 781-788) (hereinafter referred to as Non-patent Reference).
When a cascade multilevel PWM inverter connect to 6.6 kV system without transformer, the number of PWM inverter stages becomes large as shown in FIG. 11, the following problems arise. If for example 1.7 kV IGBTs (Insulated Gate Bipolar Transistor) are employed, six stages are required for each phase. When the number of inverter stages becomes large, in the first place, the number of inverter gate circuits required becomes correspondingly large, resulting in a corresponding increase in volume and circuit complexity. Also, the number of PWM switching circuits becomes large, so the total switching loss generated in the various stages becomes large. Although the number of stages can be reduced by using semiconductor devices of high withstand-voltage in the inverters, generation of heat is increased due to increased switching loss. If the switching frequency is lowered, switching loss decreases, but, since the amount of harmonics in the voltage output is increased, the filter reactor must be made large.
According to an aspect of the present technology, an object of this embodiment is to provide a small-size reactive power compensator wherein voltage harmonics can be reduced to a thoroughly satisfactory extent using cascade connected inverters, with a small number of stages, and in which transformerless interconnection with a power system can be achieved.
In order to achieve the above object, a reactive power compensator according to this embodiment is constructed as follows. Specifically, it comprises: multilevel inverter circuits respectively arranged as three-phase circuit phases; filter circuits for reducing current harmonics connected between the outputs of the multilevel inverter circuits and system interconnection terminals; and a control section for controlling the aforementioned multilevel inverter circuits and outputting prescribed three-phase AC voltage. The aforementioned multilevel inverter circuits are constituted by connecting in series two or more single-phase full-bridge single-pulse inverters that convert DC voltage to respective positive and negative single-pulse voltages once per cycle of the fundamental wave of the voltage instruction value; the terminals on the opposite side of the aforementioned multilevel inverter circuits to that of the system terminal are connected as neutral points in all phases.